1. Field of the Invention
The present invention relates to a photovoltaic element comprising a pin layer comprising non-single-crystal silicon type semiconductor material and a zinc oxide thin film layer interposed between a substrate and the pin layer. The photovoltaic element is utilized for a solar cell, photodiode, electrophotographic photosensitive member, light-emitting element and the like.
2. Related Background Art
In recent years, a study has been energetically carried out of photovoltaic elements with a zinc oxide (ZnO) thin film layer being used as a transparent conductive film. For example, an improvement in electric characteristics of fluorine-doped ZnO with a texture structure was made in "Optimization of Transparent and Reflecting Electrodes for Amorphous Silicon Solar Cells.", Gordon R G, Hu J, Musher J, Giunta C, US DOE Rep. pp. 44, 1991. In addition, a transparent conductive film having a low resistance and a high transmittance with a fluorine-added ZnO was obtained in Japanese Patent Application Laid-Open No. 1-194208.
"Research on amorphous silicon based thin film photovoltaic devices. Task B: Research on stable high efficiency, large area amorphous silicon based submodules.", Delahoy A E, Ellis F B Jr, Kampas F J, Tonon T, Weakllem H A, US DOE Rep. pp. 48, 1989 (-50 .ANG./s) reported a solar cell using an excellent high quality doped ZnO.
Studies of a solar cell using the microwave plasma CVD process (MWPCVD process) were also carried out as below: "a-Si solar cell by the microwave plasma CVD process", Azuma K, Watanabe T, Shimada H, Preprints for 50th Scientific Lecture Meeting of Applied Physics Society, pp. 566 can be mentioned. In this photovoltaic element, a good quality and high deposition rate i-layer was obtained by forming an i-layer with the MWPCVD process.
Examples of doping layer formation with the MWPCVD process can be given as follows: "High Efficiency Amorphous Solar Cell Employing ECR-CVD Produced p-Type Microcrystalline SiC Film", Y. Hattori, D. Kruangam, T. Toyama, H. Okamoto and Y. Hamakawa, Proceedings of the International PVSEC-3 Tokyo Japan 1987 pp. 171; and
"HIGH-CONDUCTIVE WIDE BAND GAP P-TYPE a-SiC:H PREPARED BY ECR CVD AND ITS APPLICATION TO HIGH EFFICIENCY a-Si BASIS SOLAR CELLS", Y. Hattori, D. Kruangam, K. Katou, Y. Nitta, H. Okamoto and Y. Hamakawa, Proceedings of 19th IEEE Photovoltaic Specialists Conference 1987 pp. 689. In these photovoltaic elements, a good quality p-layer was obtained by employing MWPCVD for production of the p-layer.
In addition, studies of a non-single-crystal silicon type semiconductor layer containing fluorine and photovoltaic elements employing it are being forwarded. For example, "Investigation into putting amorphous solar cell to practical use. Technical investigation on manufacture of high reliability amorphous solar cell elements," General situation of research and development on the sunshine plan, Solar energy 1, Light utilization technique VOL. 1985 pp.I.231-I.243, 1986;
"The chemical and configurational basis of high efficiency amorphous photovoltaic cells", Ovshinsky S R, Proceedings of 17th IEEE Photovoltaic Specialists Conference 1985 pp. 1365;
"Development of the scientific and technical basis for integrated amorphous silicon modules. Research on a-Si:F:H (B) alloys and module testing at IET-CIEMAT." Gutierrez M. T. P Delgado L, Photovolt. Power Gener., pp. 70-75, 1988; and
The effect of fluorine on the photovoltaic properties of amorphous silicon", Konagai M, Nishihata K, Takahashi K, Komoro K, Proceedings of 15th IEEE Photovoltaic Specialists Conference 1981, pp. 906. In these examples, however, mention is made of deterioration by light phenomenon and thermal stability but not of peeling-off of a semiconductor layer.
Studies of photovoltaic elements containing microcrystalline silicon have also been energetically carried out, but no mention has been made of layer separation.
U.S. Pat. No. 4,400,409 discloses a plasma CVD apparatus for continuously forming a semiconductor layer by adopting the Roll to Roll system. The photovoltaic element according to the present invention is preferably produced in series by use of such an apparatus. According to this apparatus, a plurality of depositing chambers are provided, a strip-like and elastic substrate is placed along the route through which the substrate passes the depositing chambers in succession and the substrate is continuously conveyed in the longitudinal direction while a semiconductor layer having a desired conductivity type is formed in the depositing chambers, so that a photovoltaic element with pin junctions are continually fabricated. Incidentally, in the U.S. patent, a gas gate is employed for preventing a raw material gas for incorporating into a semiconductor layer the respective valence electron controlling agent from diffusing into other depositing chambers to be mixed into other semiconductor layer. Specifically, adjacent depositing chambers are separated with a slit-like separating passage from each other and a scavenging gas such as Ar, H.sub.2 or He is allowed to flow into the respective passages separately to prevent the mutual diffusion of individual raw material gases.
This Roll-to-Roll system formation method is effective in producing a photovoltaic element such as one according to the present invention.
With the above conventional photovoltaic elements, upgrade is hoped for in preventing photo-excited carriers from recombination near the ZnO/pin layer interface or the ZnO/substrate interface. In addition, with these photovoltaic elements, improvement in open-circuit voltage and short-circuit current is also hoped for.
The effect that, when light is irradiated for a long period of time, the photoelectric conversion efficiency decreases, what is called, deterioration by light, is at issue. And, the effect that, when vibration is given for a long period of time, the photoelectric conversion efficiency decreases, what is called, deterioration by vibration, is at issue.
Furthermore, optical and deterioration by vibration at application of a bias voltage to a photovoltaic element have been at issue.
Still further, a photovoltaic element containing at least one of Ag, Al and In in the substrate has had a problem of short circuit when a bias voltage remains to be applied thereto for a long period of time.
And when a non-single-crystal silicon type semiconductor layer containing microcrystalline silicon is formed on a ZnO thin film, there has been a problem that layer peeling off is more likely to occur than observed in those containing no microcrystalline silicon.
And when a non-single-crystal silicon type semiconductor layer containing fluorine is formed on a zinc oxide thin film layer, there has been a problem that layer peeling off is more likely to occur than observed in those containing no fluorine.
In addition, when preserved over a long period of time or transported in a roll-like wound state, a ZnO thin film layer formed by the roll-to-roll process has had a problem that layer peeling off is apt to occur.